This invention relates generally to medical devices and more particularly to systems for measuring intracranial pressure, and interrogating other implanted passive resonant circuits.
Numerous patents have been issued disclosing various means for monitoring intracranial pressure by means of an implanted, passive, resonant electronic circuit that is interrogated by an external device.
U.S. Pat. No. 3,943,915 (Severson) discloses an intracranial pressure monitoring device that incorporates a lumped-constant tuned circuit. The typical Q of such a circuit is on the order of 50.
U.S. Pat. No. 4,026,276 (Chubbuck) discloses an intracranial pressure monitoring device with a lumped-constant tuned circuit. The typical Q of such a circuit is on the order of 50.
U.S. Pat. No. 4,114,606 (Seylar) discloses a monitoring device for implanted resonant circuits. They are not able to even estimate the signal-to-noise ratio, but use a "grid-dip meter" approach, i.e., the detector voltage "dips" whenever the interrogating circuit sweeps by the resonant frequency of the implanted resonant frequency circuit.
U.S. Pat. No. 4,265,252 (Chubbuck) discloses an intracranial pressure monitoring device with a lumped-constant tuned circuit.
U.S. Pat. No. 4,354,506 (Sakaguchi) discloses an intracranial pressure monitoring device with a lumped-constant tuned circuit, and purposed using a "grid-dip meter" monitoring system.
U.S. Pat. No. 5,873,840 (Neff) discloses an intracranial pressure sensor with a microwave cavity resonator. The preferred embodiment discussed includes a reflected energy measurement approach.
However, all of these devices suffer from poor signal-to-noise ratios. Thus, there remains a need for an implanted resonant circuit that provides for a response signal with good signal-to-noise ratio when interrogated.